It is already known to measure the diameter of cylindrical parts in the course of machining (in process measurement). This is the case for instance for the measuring head called "Pinvar" (registered trademark) of the Italian company Marposs. In this arrangement, the rotating cylinder is sensed by two diametrally opposed contact pieces. One such contact piece is fixed along with the measuring head. The other is movable and is located at the end of a lever pivoted on the head. This system exhibits the difficulty of using movable levers which take up space with the bearings which they require and by the sealing arrangements necessary for their good functioning. One is thus brought to a measuring head of substantial thickness, say on the order of 18 mm. Such apparatus is accordingly unsuitable for example to in-process measuring of cylindrical crankshaft bearings of small dimensions, where the available space between crank pins is reduced to 10 mm for instance.
To overcome this difficulty, this invention preferably calls for a pneumatic sensor which may be obtained in diameters not exceeding 6 mm and which is commercialized for instance by the American company Bendix, manufacturer of Sheffield products. The applicant of this invention had the idea of associating such a pneumatic sensor with an arrangement itself known including a part in the form of a V groove. When the cylinder to be measured is engaged in the groove, it is sufficient to measure the distance between the ridge of the V and the surface of the cylinder in order to deduce therefrom the diameter of the cylinder. An explanatory description of this principle will be found in the work "Taschenbuch der Langenmesstechnik", Springer Verlag, Berlin, 1954, p. 466 and 467. Such a technique has been practised and described in the patent document FR-A-2 570 487 originating from the applicant of this invention. There, however, it concerns an apparatus for static measurement of the diameters of cylindrical parts. Although the cited document foresees the possibility for the measuring head to come to bear and to center itself under the part to be measured, there is not there found however any teaching which would permit utilization of such a system on cylinders in the course of machining and enabling self-calibration of said cylinders. In particular, the presence of the grinder prevents arranging the measurement apparatus under the cylindrical part, but requires the placing of such apparatus facing the grinder, which poses certain problems not solved by the cited document, particularly that of compensating the weight of the head of the measuring apparatus as will appear in the description to follow.
The patent document DE-B-1 502 477 discloses a measuring arrangement where there exist means coupling the head to a base fixed to the frame of the machine in order to assure contact of the ridges of the head with the part to be machined, such coupling means assuring the mobility and the maintenance of the head in an X-Y plane substantially perpendicular to the axis of the part to be machined. This same document also shows elastic means urging the head against the part to be machined. It will however be noted that either the coupling means or the elastic urging means are not confined within a space bounded by the extension of the two parallel faces defining the thickness of the head. In particular, the document shows elastic urging means formed with the aid of four vertical and flexible blades. These blades are evidently located outside the space bounded by the thickness of the head. Such arrangements lead thus to a head of substantial thickness, not permitting the attainment of narrow and deep zones as is the case in crankshafts of small dimensions suggested hereinabove. Furthermore, the arrangement of the cited document does not provide any elastic sustaining means for the head, such head bearing by its own weight on the part to be machined, which tends to cause irregular wear of the contact pieces on the head. Finally, it will be mentioned that the horizontal spring blades work in compression when the head bears on the part to be machined, this requiring the use of thick blades which are thus not very flexible.
The patent document GB-A-1 099 692 also provides coupling means assuring the mobility and the maintenance of the head in an X-Y plane, as well as elastic means urging the head against the part to be machined. However, the means coupling the head to the machine frame are situated outside the thickness of the head and contribute thus to render such head wider. Furthermore, there exists no elastic sustaining means for the head, which brings about the wear suggested in the preceding paragraph.